Digital Subtraction Angiography (DSA) imaging is used in interventional medicine to diagnose vascular disease or abnormality in patients to guide the intervention, and is used subsequent to treatment to document the effectiveness of treatment. DSA is a computer-aided image processing method used to enhance vasculature images in which each pixel of data acquired in an X-ray angiography procedure is digitized. DSA relies on the comparison between images taken immediately preceding an injection of a contrast bolus (mask frames) and those obtained as the contrast bolus is passing through the target vessels (contrast frames). The mask frames are digitally subtracted from the corresponding contrast frames resulting in the contrast-filled vessels being rendered on a display free of the background detail contained in the mask frames. Additional known image processing functions for further enhancing the final images are performed to produce a series of successive images, which are then replayed sequentially, enabling a healthcare practitioner to visualize fluid flow through the target vessels.
Often, a rotation sequence of sufficient rotational coverage is acquired of the mask and contrast frames so that computed tomographic (CT) reconstruction is possible. This enables the interventional team to see the target region in 3D without the problem of overlapping structures. However, if sufficient metal is in or near the target region, such as a coil, metal artifacts occur due to the highly attenuating nature of metal leading to no signal beneath the metal and also the very high frequency content of the metal that the imaging system cannot reliably render. The metal artifacts can be severe enough to render the value of the image of the target region useless. This invention is a method to greatly reduce (if not eliminate) the metal artifacts in such imaging systems and situations.
A key step in the metal artifact reduction (MAR) is to pixel-shift a contrast frame relative to the mask (or reference) frame to align the pair of frames to match backgrounds. This also can aid the visualization of the contrast-enhanced structures in DSA images even without the CT-reconstruction. This process of translating and/or rotating the mask frames to account for movement and changes in the contrast frames is referred to as registration. In some cases, artifacts arise in processing multiple individual frames if one or more individual frames exhibit shifts in contrast-enhanced structures in a different direction than that of the background.